Multi-frequency excitation schemes for high sensitivity magnetometry measurement with drift error compensation

ABSTRACT

A system for magnetic detection includes a nitrogen vacancy (NV) diamond material, a radio frequency (RF) excitation source that provides RF excitation to the NV diamond material, an optical excitation source that provides optical excitation to the NV diamond material, an optical detector that receives an optical signal emitted by the NV diamond material, a magnetic field generator that generates a magnetic field applied to the NV diamond material, and a controller. The controller controls the RF excitation source to apply a first RF excitation having a first frequency and a second RF excitation having a second frequency. The first frequency is associated with a first slope point of a fluorescence intensity response of an NV center orientation of a first spin state, and the second frequency is associated with a second slope point of the fluorescence intensity response of the NV center orientation of the first spin state.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is related to co-pending applications, U.S. patent application Ser. No. 15/003,590, filed Jan. 21, 2016, and International Patent Application No. PCT/US2016/014336, filed Jan. 21, 2016. The contents of these applications are incorporated herein by reference in their entireties.

FIELD

The present disclosure generally relates to magnetic detection systems, and more particularly, to measurement collection schemes for a magnetic detection system.

BACKGROUND

A number of industrial applications including, but not limited to, medical devices, communication devices, and navigation systems, as well as scientific areas such as physics and chemistry can benefit from magnetic detection and imaging. Many advanced magnetic imaging systems can operate in limited conditions, for example, high vacuum and/or cryogenic temperatures, which can make them inapplicable for imaging applications that require ambient conditions. Furthermore, small size, weight and power (SWAP) magnetic sensors of moderate sensitivity, vector accuracy, and bandwidth are valuable in many applications.

Atomic-sized nitrogen-vacancy (NV) centers in diamond have been shown to have excellent sensitivity for magnetic field measurement and enable fabrication of small magnetic sensors that can readily replace existing-technology (e.g., Hall-effect) systems and devices. The sensing capabilities of diamond NV (DNV) sensors are maintained at room temperature and atmospheric pressure, and these sensors can be even used in liquid environments (e.g., for biological imaging). DNV sensing allows measurement of 3-D vector magnetic fields that is beneficial across a very broad range of applications, including communications, geological sensing, navigation, and attitude determination.

SUMMARY

According to some embodiments, a system for magnetic detection may include a nitrogen vacancy (NV) diamond material comprising a plurality of NV centers, a radio frequency (RF) excitation source configured to provide RF excitation to the NV diamond material, an optical excitation source configured to provide optical excitation to the NV diamond material, an optical detector configured to receive an optical signal emitted by the NV diamond material, a magnetic field generator configured to generate a magnetic field applied to the NV diamond material, and a controller. The controller may be configured to control the optical excitation source to apply optical excitation to the NV diamond material, control the RF excitation source to apply a first RF excitation to the NV diamond material, the first RF excitation having a first frequency, and control the RF excitation source to apply a second RF excitation to the NV diamond material, the second RF excitation having a second frequency. The first frequency may be a frequency associated with a first slope point of a fluorescence intensity response of an NV center orientation of a first spin state due to the optical excitation, the first slope point being a positive slope point, and the second frequency may be a frequency associated with a second slope point of the fluorescence intensity response of the NV center orientation of the first spin state due to the optical excitation, the second slope point being a negative slope point.

In some aspects, the controller may be configured to control the RF excitation source to alternately apply the first RF excitation as a single RF pulse and apply the second RF excitation as a single RF pulse.

In some aspects, the controller may be configured to control the RF excitation source to alternately apply the first RF excitation as two or more RF pulses in sequence and apply the second RF excitation as two or more RF pulses in sequence.

In some aspects, the controller may be configured to measure a first fluorescence intensity based on an average of a fluorescence intensity associated with each of the two or more RF pulses of the first RF excitation and measure a second fluorescence intensity based on an average of a fluorescence intensity associated with each of the two or more RF pulses of the second RF excitation.

In some aspects, the controller may be configured to control the RF excitation source to alternately apply the first RF excitation as three or more RF pulses in sequence and apply the second RF excitation as three or more RF pulses in sequence, measure a first fluorescence intensity based on an average of a fluorescence intensity associated with each of two or more RF pulses of the three or more RF pulses of the first RF excitation, measure a second fluorescence intensity based on an average of a fluorescence intensity associated with each of two or more RF pulses of the three or more RF pulses of the second RF excitation.

In some aspects, the two or more RF pulses of the first RF excitation may be applied last in the sequence of the three or more pulses, and wherein the two or more RF pulses of the second RF excitation are applied last in the sequence of the three or more pulses.

In some aspects, the positive slope point may be a maximum positive slope point of the fluorescence intensity response of the NV center orientation of the first spin state and the negative slope point may be a maximum negative slope point of the fluorescence intensity response of the NV center orientation of the first spin state.

In some aspects, the positive slope point and the negative slope point may be set as an average of a maximum positive slope point and a maximum negative slope point of the fluorescence intensity response of the NV center orientation of the first spin state due to the optical excitation.

In some aspects, the controller may be configured to measure a first fluorescence intensity at the positive slope point, measure a second fluorescence intensity at the negative slope point, and calculate a compensated fluorescence intensity based on a difference between the measured first fluorescence intensity and the measured second fluorescence intensity divided by a difference between the slope of the positive slope point and the slope of the negative slope point.

In some aspects, the controller may be configured to control the RF excitation source to apply a third RF excitation to the NV diamond material, the third RF excitation having a third frequency. The third frequency may be a frequency associated with a third slope point of the fluorescence intensity response of the NV center orientation of a second spin state due to the optical excitation.

In some aspects, the third slope point may be a positive slope point.

In some aspects, the third slope point may be a negative slope point.

According to some embodiments, a system for magnetic detection may include a nitrogen vacancy (NV) diamond material comprising a plurality of NV centers, a radio frequency (RF) excitation source configured to provide RF excitation to the NV diamond material, an optical excitation source configured to provide optical excitation to the NV diamond material, an optical detector configured to receive an optical signal emitted by the NV diamond material, a magnetic field generator configured to generate a magnetic field applied to the NV diamond material, and a controller. The controller may be configured to control the optical excitation source to apply optical excitation to the NV diamond material, control the RF excitation source to apply a first RF excitation to the NV diamond material, the first RF excitation having a first frequency, and control the RF excitation source to apply a second RF excitation to the NV diamond material, the second RF excitation having a second frequency. The first frequency may be a frequency associated with a first slope point of a fluorescence intensity response of an NV center orientation of a first spin state due to the optical excitation, and the second frequency may be a frequency associated with a second slope point of the fluorescence intensity response of the NV center orientation of a second spin state due to the optical excitation.

In some aspects, the first slope point may be a positive slope point.

In some aspects, the second slope point may be a negative slope point.

In some aspects, the first slope point may be a negative slope point.

In some aspects, the second slope point may be a negative slope point.

In some aspects, the controller may be configured to control the RF excitation source to alternately apply the first RF excitation as two or more RF pulses in sequence and apply the second RF excitation as two or more RF pulses in sequence.

In some aspects, the controller may be configured to measure a first fluorescence intensity based on an average of a fluorescence intensity associated with each of the two or more RF pulses of the first RF excitation and measure a second fluorescence intensity based on an average of a fluorescence intensity associated with each of the two or more RF pulses of the second RF excitation.

In some aspects, the controller may be configured to control the RF excitation source to alternately apply the first RF excitation as three or more RF pulses in sequence and apply the second RF excitation as three or more RF pulses in sequence, measure a first fluorescence intensity based on an average of a fluorescence intensity associated with each of two or more RF pulses of the three or more RF pulses of the first RF excitation, and measure a second fluorescence intensity based on an average of a fluorescence intensity associated with each of two or more RF pulses of the three or more RF pulses of the second RF excitation.

In some aspects, the two or more RF pulses of the first RF excitation may be applied last in the sequence of the three or more pulses, and wherein the two or more RF pulses of the second RF excitation are applied last in the sequence of the three or more pulses.

In some aspects, the controller may be configured to control the RF excitation source to apply a third RF excitation to the NV diamond material, the third RF excitation having a third frequency, and control the RF excitation source to apply a fourth RF excitation to the NV diamond material, the fourth RF excitation having a fourth frequency. The third frequency may be a frequency associated with a third slope point of the fluorescence intensity response of the NV center orientation of the first spin state due to the optical excitation, and the fourth frequency may be a frequency associated with a fourth slope point of the fluorescence intensity response of the NV center orientation of the second spin state due to the optical excitation.

According to some embodiments, a method for compensating for drift error in a magnetic detection system may include applying optical excitation to a nitrogen vacancy (NV) diamond material comprising a plurality of NV centers, applying a first RF excitation to the NV diamond material, the first RF excitation having a first frequency, applying a second RF excitation to the NV diamond material, the second RF excitation having a second frequency, applying a third RF excitation to the NV diamond material, the third RF excitation having a third frequency, and applying a fourth RF excitation to the NV diamond material, the third RF excitation having a fourth frequency. The first frequency may be a frequency associated with a first slope point of a fluorescence intensity response of an NV center orientation of a first spin state due to the optical excitation, the first slope point being a positive slope point. The second frequency may be a frequency associated with a second slope point of the fluorescence intensity response of the NV center orientation of the first spin state due to the optical excitation, the second slope point being a negative slope point. The third frequency may be a frequency associated with a third slope point of the fluorescence intensity response of the NV center orientation of a second spin state due to the optical excitation. The fourth frequency may be a frequency associated with a fourth slope point of the fluorescence intensity response of the NV center orientation of the second spin state due to the optical excitation.

In some aspects, the method may further include applying each of the steps to each of four NV center orientations of the NV diamond material.

According to some embodiments, a system for magnetic detection may include a nitrogen vacancy (NV) diamond material comprising a plurality of NV centers, a radio frequency (RF) excitation source configured to provide RF excitation to the NV diamond material, an optical excitation source configured to provide optical excitation to the NV diamond material, an optical detector configured to receive an optical signal emitted by the NV diamond material, a magnetic field generator configured to generate a magnetic field applied to the NV diamond material, a means for controlling the optical excitation source to apply optical excitation to the NV diamond material, controlling the RF excitation source to apply a first RF excitation to the NV diamond material, the first RF excitation having a first frequency, and controlling the RF excitation source to apply a second RF excitation to the NV diamond material, the second RF excitation having a second frequency. The first frequency may be a frequency associated with a first slope point of a fluorescence intensity response of an NV center orientation of a first spin state due to the optical excitation, the first slope point being a positive slope point, and the second frequency may be a frequency associated with a second slope point of the fluorescence intensity response of the NV center orientation of the first spin state due to the optical excitation, the second slope point being a negative slope point.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one orientation of an NV center in a diamond lattice.

FIG. 2 is an energy level diagram showing energy levels of spin states for an NV center.

FIG. 3 is a schematic diagram illustrating an NV center magnetic sensor system.

FIG. 4 is a graph illustrating the fluorescence as a function of an applied RF frequency of an NV center along a given direction for a zero magnetic field.

FIG. 5 is a graph illustrating the fluorescence as a function of an applied RF frequency for four different NV center orientations for a non-zero magnetic field.

FIG. 6 is a schematic diagram illustrating a magnetic field detection system according to some embodiments.

FIG. 7A is a graph illustrating fluorescence reduction as a function of an applied RF frequency for a positive spin state of an NV center orientation.

FIG. 7B is a graph illustrating fluorescence reduction as a function of an applied RF frequency for a negative spin state of the NV center orientation of FIG. 7A.

FIG. 8A illustrates a measurement collection scheme for vertical drift error compensation according to some embodiments.

FIG. 8B shows a measurement collection scheme for vertical drift error compensation according to some embodiments.

FIG. 8C shows a measurement collection scheme for horizontal drift error compensation according to some embodiments.

DETAILED DESCRIPTION

Measurement errors due to vertical and horizontal fluctuations in fluorescence intensity caused by internal and external effects of the system (e.g., optical excitation, thermal and/or strain effects) may be addressed in a magnetic detection system including multi-RF excitation. Fluorescence intensity measurements may be obtained at resonant frequencies associated with the positive and negative maximum (including greatest and near greatest) slope points of a response curve of an NV center orientation and spin state (m_(s)=+1) to account for vertical drift error. In addition, fluorescence intensity measurements may be obtained at resonant frequencies associated with the positive and/or negative maximum (including greatest and near greatest) slope points of the response curves of an NV center orientation at both spin states (m_(s)=+1 and m_(s)=−1) to account for horizontal drift error. By compensating for such errors, the system may realize increased sensitivity and stability when calculating an external magnetic field acting on the system. In certain embodiments, guard intervals, in the form of multi-pulse sets of RF excitation at a given resonant frequency, and/or guard pulses, in the form of initial pulses used to stabilize the system without providing measurement data, may also be utilized during the collection process to allow for sufficient repolarization of the system when switching between resonant frequencies. Such guard intervals and/or guard pulses may ensure that residual effects due to previous measurement collections are reduced or eliminated. Among other things, this allows the system to forego the use of high-powered optical excitation for repolarization, thus improving sensor performance and cost.

The NV Center, Its Electronic Structure, and Optical and RF Interaction

The NV center in a diamond comprises a substitutional nitrogen atom in a lattice site adjacent a carbon vacancy as shown in FIG. 1. The NV center may have four orientations, each corresponding to a different crystallographic orientation of the diamond lattice.

The NV center may exist in a neutral charge state or a negative charge state. The neutral charge state uses the nomenclature NV⁰, while the negative charge state uses the nomenclature NV.

The NV center has a number of electrons, including three unpaired electrons, each one from the vacancy to a respective of the three carbon atoms adjacent to the vacancy, and a pair of electrons between the nitrogen and the vacancy. The NV center, which is in the negatively charged state, also includes an extra electron.

The NV center has rotational symmetry, and as shown in FIG. 2, has a ground state, which is a spin triplet with ³A₂ symmetry with one spin state m_(s)=0, and two further spin states m_(s)=+1, and m_(s)=−1. In the absence of an external magnetic field, the m_(s)=±1 energy levels are offset from the m_(s)=0 due to spin-spin interactions, and the m_(s)=±1 energy levels are degenerate, i.e., they have the same energy. The m_(s)=0 spin state energy level is split from the m_(s)=±1 energy levels by an energy of 2.87 GHz for a zero external magnetic field.

Introducing an external magnetic field with a component along the NV axis lifts the degeneracy of the m_(s)=±1 energy levels, splitting the energy levels m_(s)=±1 by an amount 2gμ_(B)Bz, where g is the g-factor, μ_(B) is the Bohr magneton, and Bz is the component of the external magnetic field along the NV axis. This relationship is correct to a first order and inclusion of higher order corrections is a straightforward matter and will not materially affect the computational and logic steps.

The NV center electronic structure further includes an excited triplet state ³E with corresponding m_(s)=0 and m_(s)=±1 spin states. The optical transitions between the ground state ³A₂ and the excited triplet ³E are predominantly spin conserving, meaning that the optical transitions are between initial and final states that have the same spin. For a direct transition between the excited triplet ³E and the ground state ³A₂, a photon of red light is emitted with a photon energy corresponding to the energy difference between the energy levels of the transitions.

An alternative non-radiative decay route from the triplet ³E to the ground state ³A₂ via intermediate electron states exists, in which the intermediate states are thought to be intermediate singlet states A, E with intermediate energy levels. The transition rate from the m_(s)=±1 spin states of the excited triplet ³E to the intermediate energy levels is significantly greater than the transition rate from the m_(s)=0 spin state of the excited triplet ³E to the intermediate energy levels. The transition from the singlet states A, E to the ground state triplet ³A₂ predominantly decays to the m_(s)=0 spin state over the m_(s)=±1 spins states. These features of the decay from the excited triplet ³E state via the intermediate singlet states A, E to the ground state triplet ³A allows that, if optical excitation is provided to the system, the optical excitation will eventually pump the NV center into the m_(s)=0 spin state of the ground state ³A₂. In this way, the population of the m_(s)=0 spin state of the ground state ³A₂ may be reset to a maximum polarization determined by the decay rates from the triplet ³E to the intermediate singlet states.

Another feature of the decay is that the fluorescence intensity due to optically stimulating the excited triplet ³E state is less for the m_(s)=±1 states than for the m_(s)=0 spin state. This is so because the decay via the intermediate states does not result in a photon emitted in the fluorescence band, and because of the greater probability that the m_(s)=±1 states of the excited triplet ³E state will decay via the non-radiative decay path. The lower fluorescence intensity for the m_(s)=±1 states than for the m_(s)=0 spin state allows the fluorescence intensity to be used to determine the spin state. As the population of the m_(s)=±1 states increases relative to the m_(s)=0 spin, the overall fluorescence intensity will be reduced.

The NV Center, or Magneto-Optical Defect Center, Magnetic Sensor System

FIG. 3 is a schematic diagram illustrating an NV center magnetic sensor system 300 that uses fluorescence intensity to distinguish the m_(s)=±1 states, and to measure the magnetic field based on the energy difference between the m_(s)=+1 state and the m_(s)=−1 state. The system 300 includes an optical excitation source 310, which directs optical excitation to an NV diamond material 320 with NV centers. The system further includes an RF excitation source 330, which provides RF radiation to the NV diamond material 320. Light from the NV diamond may be directed through an optical filter 350 to an optical detector 340.

The RF excitation source 330 may be a microwave coil, for example. The RF excitation source 330, when emitting RF radiation with a photon energy resonant with the transition energy between ground m_(s)=0 spin state and the m_(s)=+1 spin state, excites a transition between those spin states. For such a resonance, the spin state cycles between ground m_(s)=0 spin state and the m_(s)=+1 spin state, reducing the population in the m_(s)=0 spin state and reducing the overall fluorescence at resonances. Similarly, resonance occurs between the m_(s)=0 spin state and the m_(s)=−1 spin state of the ground state when the photon energy of the RF radiation emitted by the RF excitation source is the difference in energies of the m_(s)=0 spin state and the m_(s)=−1 spin state, or between the m_(s)=0 spin state and the m_(s)=+1 spin state, there is a decrease in the fluorescence intensity.

The optical excitation source 310 may be a laser or a light emitting diode, for example, which emits light in the green, for example. The optical excitation source 310 induces fluorescence in the red, which corresponds to an electronic transition from the excited state to the ground state. Light from the NV diamond material 320 is directed through the optical filter 350 to filter out light in the excitation band (in the green, for example), and to pass light in the red fluorescence band, which in turn is detected by the detector 340. The optical excitation light source 310, in addition to exciting fluorescence in the diamond material 320, also serves to reset the population of the m_(s)=0 spin state of the ground state ³A₂ to a maximum polarization, or other desired polarization.

For continuous wave excitation, the optical excitation source 310 continuously pumps the NV centers, and the RF excitation source 330 sweeps across a frequency range that includes the zero splitting (when the m_(s)=±1 spin states have the same energy) photon energy of 2.87 GHz. The fluorescence for an RF sweep corresponding to a diamond material 320 with NV centers aligned along a single direction is shown in FIG. 4 for different magnetic field components Bz along the NV axis, where the energy splitting between the m_(s)=−1 spin state and the m_(s)=+1 spin state increases with Bz. Thus, the component Bz may be determined. Optical excitation schemes other than continuous wave excitation are contemplated, such as excitation schemes involving pulsed optical excitation, and pulsed RF excitation. Examples of pulsed excitation schemes include Ramsey pulse sequence and spin echo pulse sequence.

In general, the diamond material 320 will have NV centers aligned along directions of four different orientation classes. FIG. 5 illustrates fluorescence as a function of RF frequency for the case where the diamond material 320 has NV centers aligned along directions of four different orientation classes. In this case, the component Bz along each of the different orientations may be determined. These results, along with the known orientation of crystallographic planes of a diamond lattice, provide not only the magnitude of the external magnetic field to be determined, but also the direction of the magnetic field.

While FIG. 3 illustrates an NV center magnetic sensor system 300 with NV diamond material 320 with a plurality of NV centers, in general, the magnetic sensor system may instead employ a different magneto-optical defect center material, with a plurality of magneto-optical defect centers. The electronic spin state energies of the magneto-optical defect centers shift with magnetic field, and the optical response, such as fluorescence, for the different spin states is not the same for all of the different spin states. In this way, the magnetic field may be determined based on optical excitation, and possibly RF excitation, in a corresponding way to that described above with NV diamond material.

FIG. 6 is a schematic diagram of a system 600 for a magnetic field detection system according to some embodiments. The system 600 includes an optical excitation source 610, which directs optical excitation to an NV diamond material 620 with NV centers, or another magneto-optical defect center material with magneto-optical defect centers. An RF excitation source 630 provides RF radiation to the NV diamond material 620. A magnetic field generator 670 generates a magnetic field, which is detected at the NV diamond material 620.

The magnetic field generator 670 may generate magnetic fields with orthogonal polarizations, for example. In this regard, the magnetic field generator 670 may include two or more magnetic field generators, such as two or more Helmholtz coils or other magnetic electronic devices. The two or more magnetic field generators may be configured to provide a magnetic field having a predetermined direction, each of which provide a relatively uniform magnetic field at the NV diamond material 620. The predetermined directions may be orthogonal to one another. In addition, the two or more magnetic field generators of the magnetic field generator 670 may be disposed at the same position, or may be separated from each other. In the case that the two or more magnetic field generators are separated from each other, the two or more magnetic field generators may be arranged in an array, such as a one-dimensional or two-dimensional array, for example.

The system 600 may be arranged to include one or more optical detection systems 605, where each of the optical detection systems 605 includes the optical detector 640, optical excitation source 610, and NV diamond material 620. Furthermore, the magnetic field generator 670 may have a relatively high power as compared to the optical detection systems 605. In this way, the optical systems 605 may be deployed in an environment that requires a relatively lower power for the optical systems 605, while the magnetic field generator 670 may be deployed in an environment that has a relatively high power available for the magnetic field generator 670 so as to apply a relatively strong magnetic field.

The system 600 further includes a controller 680 arranged to receive a light detection signal from the optical detector 640 and to control the optical excitation source 610, the RF excitation source 630, and the second magnetic field generator 675. The controller may be a single controller, or multiple controllers. For a controller including multiple controllers, each of the controllers may perform different functions, such as controlling different components of the system 600. The second magnetic field generator 675 may be controlled by the controller 680 via an amplifier 660, for example.

The RF excitation source 630 may be a microwave coil, for example. The RF excitation source 630 is controlled to emit RF radiation with a photon energy resonant with the transition energy between the ground m_(s)=0 spin state and the m_(s)=±1 spin states as discussed above with respect to FIG. 3.

The optical excitation source 610 may be a laser or a light emitting diode, for example, which emits light in the green, for example. The optical excitation source 610 induces fluorescence in the red from the NV diamond material 620, where the fluorescence corresponds to an electronic transition from the excited state to the ground state. Light from the NV diamond material 620 is directed through the optical filter 650 to filter out light in the excitation band (in the green, for example), and to pass light in the red fluorescence band, which in turn is detected by the optical detector 640. The optical excitation light source 610, in addition to exciting fluorescence in the NV diamond material 620, also serves to reset the population of the m_(s)=0 spin state of the ground state ³A₂ to a maximum polarization, or other desired polarization.

The controller 680 is arranged to receive a light detection signal from the optical detector 640 and to control the optical excitation source 610, the RF excitation source 630, and the second magnetic field generator 675. The controller may include a processor 682 and a memory 684, in order to control the operation of the optical excitation source 610, the RF excitation source 630, and the second magnetic field generator 675. The memory 684, which may include a nontransitory computer readable medium, may store instructions to allow the operation of the optical excitation source 610, the RF excitation source 630, and the second magnetic field generator 675 to be controlled. That is, the controller 680 may be programmed to provide control.

Measurement Collection Process

According to certain embodiments, the controller 680 controls the operation of the optical excitation source 610, the RF excitation source 630, and the magnetic field generator 670 to perform Optically Detected Magnetic Resonance (ODMR). Specifically, the magnetic field generator 670 may be used to apply a bias magnetic field that sufficiently separates the intensity responses for each of the four NV center orientations. The controller 680 then controls the optical excitation source 610 to provide optical excitation to the NV diamond material 620 and the RF excitation source 630 to provide RF excitation to the NV diamond material 620. The resulting fluorescence intensity responses for each of the NV axes are collected over time to determine the components of the external magnetic field Bz aligned along directions of the four NV center orientations of the NV diamond material 620, which may then be used to calculate the estimated vector magnetic field acting on the system 600. The excitation scheme utilized during the measurement collection process (i.e., the applied optical excitation and the applied RF excitation) may be any appropriate excitation scheme. For example, the excitation scheme may utilize continuous wave (CW) magnetometry, pulsed magnetometry, and variations on CW and pulsed magnetometry (e.g., pulsed RF excitation with CW optical excitation). In cases where Ramsey pulse RF sequences are used, pulse parameters π and τ may be optimized using Rabi analysis and FID-Tau sweeps prior to the collection process, as described in, for example, U.S. patent application Ser. No. 15/003,590.

During the measurement collection process, fluctuations may occur in the measured intensity response due to effects caused by components of the system 600, rather than due to true changes in the external magnetic field. For example, prolonged optical excitation of the NV diamond material by the optical excitation source 610 may cause vertical (e.g., red photoluminescence intensity) fluctuations, or vertical drift, in the intensity response, causing the response curve to shift upward or downward over time. In addition, thermal effects within the system 600 may result in horizontal (e.g., frequency) fluctuations, or horizontal drift, in the measured intensity response, causing the response curve to translate left or right over time.

In some systems, the excitation scheme is configured such that the measurement collection process occurs at a single resonant frequency associated with a given spin state (e.g., m_(s)=+1) of an NV center orientation. This resonant frequency may be either the frequency associated with the positive maximum slope point or the frequency associated with the negative maximum slope point of the response curve. Intensity response changes that occur at the particular frequency are tracked and used to determine changes in the external magnetic field Bz. However, because these measurement techniques utilize data at only a single point of the response curve (e.g., the positive maximum slope point or the negative maximum slope point), it can be difficult to account for those changes in the intensity response that are not due to the external magnetic field Bz, but are rather due to internal or external system effects. For example, when only a single RF frequency is tracked for measurement purposes, vertical drift due to prolonged optical excitation and horizontal drift due to thermal effects may be perceived as changes in the external magnetic field Bz, thus introducing error into the estimated vector magnetic field. Thus, compensation for these internal errors during the measurement collection process is desirable to maximize sensitivity and stability of the magnetic detection system 600.

Vertical Drift and Horizontal Drift Error Compensation

FIG. 7A illustrates one example of a reduced fluorescence intensity response associated with a particular NV axis orientation and a first spin state (e.g., m_(s)=+1). The graph shown in FIG. 7A is a zoomed-in view of the signal of interest (e.g., the particular NV axis orientation at the first spin state) via an offset and gain within the optical detector 640 and related circuitry of the system 600. As shown in FIG. 7A, the intensity response curve for the given spin state includes two maximum (including greatest and near greatest) slope points, a positive maximum (including greatest and near greatest) slope point 12A and a negative maximum (including greatest and near greatest) slope point 12B.

To compensate for vertical drift error, data is collected on both the positive maximum slope point 12A and the negative maximum slope point 12B during a collection process for a given magnetometry response curve. In some embodiments, however, data may be collected on a positive slope point 12A and a negative slope point 12B that is the average between the positive maximum slope and the negative maximum slope for a given response curve to allow for faster switching between relative frequencies during measurement collection.

By collecting data on both the positive slope point 12A and the negative slope point 12B for a response curve, changes due to vertical drift may be detected and accounted for during the external magnetic field calculation process. For example, if a shift in the response curve is due to a true change in the external magnetic field, the intensity response associated with the slope point 12A and the intensity response associated with the slope point 12B should shift in opposite directions (e.g., the intensity response associated with the slope point 12A increases, while the intensity response associated with the slope point 12B decreases, or vice versa). On the other hand, if a shift in the response curve is due to internal system factors that may cause vertical fluctuations, the intensity response associated with the slope points 12A, 12B should shift in equal directions (e.g., the intensity responses for slope points 12A, 12B both increase). Thus, by determining the relative shift in intensity response of slope points 12A, 12B of the response curve, error due to vertical drift may be detected. The resulting intensity measurements of the positive slope point 12A and the negative slope point 12B are then subtracted and divided by the difference of the slopes 12A, 12B (i.e., positive slope 12A−negative slope 12B≈2*positive slope 12A), allowing for compensation of vertical fluctuations associated with vertical drift. In some embodiments, the vertical compensation process provides similar sensitivity as compared to a single RF frequency data collection process, described above, but reduces the bandwidth of the collection process by a factor of two.

FIG. 7B illustrates the reduced fluorescence intensity response associated with the same NV axis orientation shown in FIG. 7A and a second spin state (e.g., m_(s)=−1), which is opposite to the first spin state. Like FIG. 7A, FIG. 7B shows a zoomed-in view of the signal of interest (e.g., the particular NV axis orientation at the second spin state) via an offset and gain within the optical detector 640 and related circuitry of the system 600. Similar to the vertical drift compensation process, horizontal drift may be compensated by performing data collection on two different slope points. In this case, data is collected on a first slope point associated with the first spin state shown in FIG. 7A and a second slope point associated with the second spin state shown in FIG. 7B. The first slope point and the second slope point may be selected independently of each other. For example, in some embodiments, the first slope point and the second slope point have equal signs (i.e., positive slope points 12A, 12A′ or negative slope points 12B, 12B′). In other embodiments, however, the first slope point and the second slope point may have opposite signs (e.g., slope points 12A, 12B′ or slope points 12B, 12A′). By collecting measurement data associated with maximum slope points of the two spin states of a given NV axis orientation, horizontal drift error may be estimated and accounted for in magnetic field calculations. For example, if a shift in the intensity response is due to changes in the external magnetic field acting on the system 600, the response curves associated with each of the spin states should shift relative to one another (i.e., either outward or inward relative to the zero splitting frequency). If, on the other hand, a shift in the intensity response is due to thermal effects within the system 600, the response curves associated with each of the spin states translate. Thus, like vertical drift compensation, horizontal shifts due to internal thermal effects may be determined and compensated during the collection process.

In certain embodiments, the measurement collection process may include both vertical drift error compensation and horizontal drift error compensation by switching between frequencies associated with the positive and negative slopes of a response curve for the first spin state and a frequency associated with a slope point of a response curve for the second spin state of an NV center orientation, allowing for magnetometry calculations that account for both vertical drift and horizontal drift due to internal components of the system 600. In addition, while processing for the compensation of vertical drift and/or horizontal drift may occur at the relative fluorescence intensity level, as described above, error due to both effects may be compensated during processing associated with the external magnetic field Bz estimation.

Measurement Collection Schemes

When switching between frequencies of a given NV center orientation and/or spin state, fluorescence dimming from a previous frequency may impact the measurement data collected on a subsequent frequency. Optical excitation power is often increased to reduce the time required to allow the system to repolarize to mitigate this effect. However, such a solution increases costs in terms of sensor SWAP, RF power, thermal stability, sensor complexity, and achievable sensitivity. As such, to ensure sufficient repolarization of the system 600 when shifting measurement collection to a different frequency without significantly increasing the costs associated with the system 600, guard intervals and/or guard pulses may be utilized during the measurement collection process, as shown in FIGS. 8A-8C. By utilizing guard intervals and/or pulses between measurement collections at different frequencies, measurement information from a given NV center orientation or spin state impacting the measurement of subsequent orientations and/or spin states due to residual dimming may be avoided. Moreover, because guard intervals/pulses reduce the effective sensor level duty cycle, multi-pulse coherent integration schemes may be used to further optimize magnetometry performance.

FIG. 8A shows one example of a measurement collection scheme in which error due to vertical drift is compensated through alternating single pulse intervals of data collection 20 on a first slope point (e.g., positive slope point 12A) of a response curve (indicated by solid lines) and data collection 25 on the second slope point (e.g., negative slope point 12B) of the response curve (indicated by dashed lines). In this case, a faster net sample rate may be achieved through constant switching between the two slope points 20, 25. The measurement collection scheme shown in FIG. 8A may be similarly applied for RF schemes utilizing horizontal drift error compensation.

In certain embodiments, to further reduce the impact of residual noise, longer data collection intervals may be used, such as the measurement collection scheme shown in FIGS. 8B and 8C. As shown in FIG. 8B, error due to vertical drift is compensated through alternating multi-pulse data collection interval 30 a-30 e on the first slope point (e.g., positive slope point 12A) of the response curve (indicated by solid lines) and multi-pulse data collection interval 35 a-35 e on the second slope point (e.g., negative slope point 12B) of the response curve (indicated by dashed liens). Similarly, as shown in FIG. 8C, error due to horizontal drift is compensated through alternating multi-pulse data collection 40 a-40 e (indicated by solid lines) on a first slope point of the response curve associated with a first spin state (e.g., positive slope point 12A) and multi-pulse data collection 45 a-45 e (indicated by dashed lines) on a second slope point of the response curve associated with a second spin state (e.g., positive slope point 12A′) of the response curve.

While five pulses are shown for each data collection interval in FIGS. 8B and 8C, the total number of pulses or windows may vary and range from one pulse per interval up to about 400 pulses per interval. Longer segments of data collection allow for the averaging of intensity measurements over 60 Hz cycles, which provides a low-pass filter that nulls harmonics due to outside noise. In addition, in some embodiments, each of the pulses in a data collection interval (e.g., pulses 30 a-30 e shown in FIG. 8B) may be averaged to achieve a better signal-to-noise ratio. In other embodiments, initial pulses in a data collection interval (e.g., pulses 30 a-30 c shown in FIG. 8B) may also serve as guard “pulses,” in which only the subsequent pulses (e.g., pulses 30 d-30 e) are averaged to obtain measurement data. These guard pulses allow for the thermal stability of the system 600 to be maintained by maintaining a regular RF excitation and optical excitation pattern while allowing the system 600 to ignore intensity measurements associated with transitions between frequencies.

In some cases, the need for guard intervals and/or guard pulses to ensure sufficient repolarization of the system 600 may be eliminated through the use of two optical light sources, one with a relatively high power to provide reset of spin polarization and another to induce fluorescence for the readout. Such a system is described in U.S. Provisional Patent Application No. 62/343,600, filed May 31, 2016, which is incorporated herein by reference in its entirety.

In addition to guard intervals and/or guard pulses, in cases of RF excitation applied as Ramsey RF pulse sequences, the pulse sequence parameters may be re-optimized (i.e., pulse parameters π and τ) when switching from a response curve associated with one NV center orientation and/or spin state to a response curve associated with another NV center orientation and/or spin state. For example, when switching from a response curve associated with a first spin state of an NV center orientation to a response curve associated with a second spin state of the same NV center orientation, such as during horizontal drift error compensation, the Ramsey pulse sequence parameters may be re-optimized for the response curve associated with the second spin state. By doing so, the fluorescence intensity values and the contrast values may better match between the two response curves, thereby ensuring maximum sensitivity during the measurement collection process.

Some concepts presented herein provide for a magnetic detection system that provides for a multi-RF excitation scheme capable of compensating for measurement errors due to vertical and horizontal fluctuations in fluorescence intensity during the collection process, allowing for increased sensitivity and stability of the detection system. In addition, by utilizing guard intervals (i.e., multi-pulse sets) while switching between frequencies and guard pulses within pulse sets ensures that residual effects due to previous measurement collections are reduced or eliminated. This allows a system to forego the use of high-powered optical excitation for the required repolarization of the system, thus improving sensor performance and cost.

Embodiments have been described in detail with particular reference to preferred embodiments, but it will be understood by those skilled in the art that variations and modifications can be effected within the spirit and scope of this disclosure. 

1. A system for magnetic detection, comprising: a magneto-optical defect center material comprising a plurality of magneto-optical defect centers; a radio frequency (RF) excitation source configured to provide RF excitation to the magneto-optical defect center material; an optical excitation source configured to provide optical excitation to the magneto-optical defect center material; an optical detector configured to receive an optical signal emitted by the magneto-optical defect center material; a magnetic field generator configured to generate a magnetic field applied to the magneto-optical defect center material; and a controller configured to: control the optical excitation source to apply optical excitation to the magneto-optical defect center material; control the RF excitation source to apply a first RF excitation to the magneto-optical defect center material, the first RF excitation having a first frequency; and control the RF excitation source to apply a second RF excitation to the magneto-optical defect center material, the second RF excitation having a second frequency, wherein the first frequency is a frequency associated with a first slope point of a fluorescence intensity response of a magneto-optical defect center orientation of a first spin state due to the optical excitation, the first slope point being a positive slope point, and wherein the second frequency is a frequency associated with a second slope point of the fluorescence intensity response of the magneto-optical defect center orientation of the first spin state due to the optical excitation, the second slope point being a negative slope point.
 2. The system of claim 1, wherein the controller is configured to control the RF excitation source to alternately apply the first RF excitation as a single RF pulse and apply the second RF excitation as a single RF pulse.
 3. The system of claim 1, wherein the controller is configured to control the RF excitation source to alternately apply the first RF excitation as two or more RF pulses in sequence and apply the second RF excitation as two or more RF pulses in sequence.
 4. The system of claim 3, wherein the controller is configured to measure a first fluorescence intensity based on an average of a fluorescence intensity associated with each of the two or more RF pulses of the first RF excitation and measure a second fluorescence intensity based on an average of a fluorescence intensity associated with each of the two or more RF pulses of the second RF excitation.
 5. The system of claim 1, wherein the controller is further configured to: control the RF excitation source to alternately apply the first RF excitation as three or more RF pulses in sequence and apply the second RF excitation as three or more RF pulses in sequence; measure a first fluorescence intensity based on an average of a fluorescence intensity associated with each of two or more RF pulses of the three or more RF pulses of the first RF excitation; and measure a second fluorescence intensity based on an average of a fluorescence intensity associated with each of two or more RF pulses of the three or more RF pulses of the second RF excitation.
 6. The system of claim 5, wherein the two or more RF pulses of the first RF excitation are applied last in the sequence of the three or more pulses, and wherein the two or more RF pulses of the second RF excitation are applied last in the sequence of the three or more pulses.
 7. The system of claim 1, wherein the positive slope point is a maximum positive slope point of the fluorescence intensity response of the magneto-optical defect center orientation of the first spin state and the negative slope point is a maximum negative slope point of the fluorescence intensity response of the magneto-optical defect center orientation of the first spin state.
 8. The system of claim 1, wherein the positive slope point and the negative slope point are set as an average of a maximum positive slope point and a maximum negative slope point of the fluorescence intensity response of the magneto-optical defect center orientation of the first spin state due to the optical excitation.
 9. The system of claim 1, wherein the controller is further configured to: measure a first fluorescence intensity at the positive slope point; measure a second fluorescence intensity at the negative slope point; and calculate a compensated fluorescence intensity based on a difference between the measured first fluorescence intensity and the measured second fluorescence intensity divided by a difference between the slope of the positive slope point and the slope of the negative slope point.
 10. The system of claim 1, wherein the controller is further configured to: control the RF excitation source to apply a third RF excitation to the magneto-optical defect center material, the third RF excitation having a third frequency, wherein the third frequency is a frequency associated with a third slope point of the fluorescence intensity response of the magneto-optical defect center orientation of a second spin state due to the optical excitation.
 11. The system of claim 10, wherein the third slope point is a positive slope point.
 12. The system of claim 10, wherein the third slope point is a negative slope point.
 13. A system for magnetic detection, comprising: a magneto-optical defect center material comprising a plurality of magneto-optical defect centers; a radio frequency (RF) excitation source configured to provide RF excitation to the material; an optical excitation source configured to provide optical excitation to the magneto-optical defect center material; an optical detector configured to receive an optical signal emitted by the magneto-optical defect center material; a magnetic field generator configured to generate a magnetic field applied to the magneto-optical defect center material; and a controller configured to: control the optical excitation source to apply optical excitation to the magneto-optical defect center material; control the RF excitation source to apply a first RF excitation to the magneto-optical defect center material, the first RF excitation having a first frequency; and control the RF excitation source to apply a second RF excitation to the magneto-optical defect center material, the second RF excitation having a second frequency, wherein the first frequency is a frequency associated with a first slope point of a fluorescence intensity response of a magneto-optical defect center orientation of a first spin state due to the optical excitation, and wherein the second frequency is a frequency associated with a second slope point of the fluorescence intensity response of the magneto-optical defect center orientation of a second spin state due to the optical excitation.
 14. The system of claim 13, wherein the first slope point is a positive slope point.
 15. The system of claim 14, wherein the second slope point is a negative slope point.
 16. The system of claim 14, wherein the second slope point is a positive slope point.
 17. The system of claim 13, wherein the first slope point is a negative slope point.
 18. The system of claim 17, wherein the second slope point is a positive slope point.
 19. The system of claim 17, wherein the second slope point is a negative slope point.
 20. The system of claim 13, wherein the controller is configured to control the RF excitation source to alternately apply the first RF excitation as two or more RF pulses in sequence and apply the second RF excitation as two or more RF pulses in sequence.
 21. The system of claim 20, wherein the controller is configured to measure a first fluorescence intensity based on an average of a fluorescence intensity associated with each of the two or more RF pulses of the first RF excitation and measure a second fluorescence intensity based on an average of a fluorescence intensity associated with each of the two or more RF pulses of the second RF excitation.
 22. The system of claim 13, wherein the controller is further configured to: control the RF excitation source to alternately apply the first RF excitation as three or more RF pulses in sequence and apply the second RF excitation as three or more RF pulses in sequence; measure a first fluorescence intensity based on an average of a fluorescence intensity associated with each of two or more RF pulses of the three or more RF pulses of the first RF excitation; and measure a second fluorescence intensity based on an average of a fluorescence intensity associated with each of two or more RF pulses of the three or more RF pulses of the second RF excitation.
 23. The system of claim 22, wherein the two or more RF pulses of the first RF excitation are applied last in the sequence of the three or more pulses, and wherein the two or more RF pulses of the second RF excitation are applied last in the sequence of the three or more pulses.
 24. The system of claim 13, wherein the controller is further configured to: control the RF excitation source to apply a third RF excitation to the magneto-optical defect center material, the third RF excitation having a third frequency; and control the RF excitation source to apply a fourth RF excitation to the magneto-optical defect center material, the fourth RF excitation having a fourth frequency, wherein the third frequency is a frequency associated with a third slope point of the fluorescence intensity response of the magneto-optical defect center orientation of the first spin state due to the optical excitation, and wherein the fourth frequency is a frequency associated with a fourth slope point of the fluorescence intensity response of the magneto-optical defect center orientation of the second spin state due to the optical excitation.
 25. A method for compensating for drift error in a magnetic detection system, the method comprising: applying optical excitation to a magneto-optical defect center material comprising a plurality of magneto-optical defect centers; applying a first RF excitation to the magneto-optical defect center material, the first RF excitation having a first frequency; applying a second RF excitation to the magneto-optical defect center material, the second RF excitation having a second frequency; applying a third RF excitation to the magneto-optical defect center material, the third RF excitation having a third frequency; and applying a fourth RF excitation to the magneto-optical defect center material, the third RF excitation having a fourth frequency, wherein the first frequency is a frequency associated with a first slope point of a fluorescence intensity response of a magneto-optical defect center orientation of a first spin state due to the optical excitation, the first slope point being a positive slope point, wherein the second frequency is a frequency associated with a second slope point of the fluorescence intensity response of the magneto-optical defect center orientation of the first spin state due to the optical excitation, the second slope point being a negative slope point, wherein the third frequency is a frequency associated with a third slope point of the fluorescence intensity response of the magneto-optical defect center orientation of a second spin state due to the optical excitation, and wherein the fourth frequency is a frequency associated with a fourth slope point of the fluorescence intensity response of the magneto-optical defect center orientation of the second spin state due to the optical excitation.
 26. The method of claim 25, further comprising applying each of the steps to each of four magneto-optical defect center orientations of the magneto-optical defect center material.
 27. A system for magnetic detection, comprising: a magneto-optical defect center material comprising a plurality of magneto-optical defect centers; a radio frequency (RF) excitation source configured to provide RF excitation to the magneto-optical defect center material; an optical excitation source configured to provide optical excitation to the magneto-optical defect center material; an optical detector configured to receive an optical signal emitted by the magneto-optical defect center material; a magnetic field generator configured to generate a magnetic field applied to the magneto-optical defect center; and a means for: controlling the optical excitation source to apply optical excitation to the magneto-optical defect center material; controlling the RF excitation source to apply a first RF excitation to the magneto-optical defect center material, the first RF excitation having a first frequency; and controlling the RF excitation source to apply a second RF excitation to the magneto-optical defect center material, the second RF excitation having a second frequency, wherein the first frequency is a frequency associated with a first slope point of a fluorescence intensity response of a magneto-optical defect center orientation of a first spin state due to the optical excitation, the first slope point being a positive slope point, and wherein the second frequency is a frequency associated with a second slope point of the fluorescence intensity response of the magneto-optical defect center orientation of the first spin state due to the optical excitation, the second slope point being a negative slope point.
 28. The system of claim 1, wherein the magneto-optical defect center material is a nitrogen vacancy (NV) diamond material.
 29. The system of claim 13, wherein the magneto-optical defect center material is a nitrogen vacancy (NV) diamond material.
 30. The method of claim 25, wherein the magneto-optical defect center material is a nitrogen vacancy (NV) diamond material.
 31. The system of claim 27, wherein the magneto-optical defect center material is a nitrogen vacancy (NV) diamond material. 